This invention relates to quadrature demodulation of an angular modulated signal which is derived by angular modulation of a carrier signal by a modulating signal comprising symbols representative of binary one and zero and is received by a receiver as a received signal. The modulating signal comprises a particular symbol sequence or pattern comprising the symbols in a predetermined manner and a data symbol sequence comprising the symbols to represent data or information which is to be transmitted to the receiver. The particular symbol sequence may be, for example, a preamble used in time division multiple access communication.
More particularly describing the predetermined manner, the symbols are included in the particular symbol sequence at a predetermined symbol interval. The particular symbol sequence consists of a predetermined number of symbols representative of all the binary ones or all the binary zeros. Alternatively, the particular symbol sequence may include a partial sequence in which the symbols alternatingly represent the binary one and zero. Such a predetermined manner is preliminarily known at the receiver side. Incidentally, the symbols are included in the data symbol sequence also at the predetermined symbol interval.
When the angular modulated or the received signal is quadrature demodulated, a demodulated signal is produced to comprise inphase and quadrature phase components. In this connection, it may be mentioned that the received signal comprises a particular signal sequence and a data signal sequence which are produced by angular modulating a carrier signal with the particular and the data symbol sequences. The demodulated signal therefore comprises a reproduced symbol sequence and a reproduced data sequence. The data signal sequence may briefly be called a data sequence with omission of the word "signal".
In order so to quadrature demodulate the received signal, a reference carrier signal is used in the receiver. The reference carrier signal is either recovered from the received signal or locally generated by a carrier oscillator. In either event, the reference carrier signal must have a local carrier frequency which is equal to a received carrier frequency of the received carrier signal. Various methods are known to keep the local carrier frequency equal to the received carrier frequency. In whichever of the methods, the local carrier recovery (oscillation) circuit must have a complicated structure. When used, the local carrier recovery circuit must be operable with a high precision and stability. The local carrier recovery circuit therefore becomes bulky and expensive. Otherwise, the particular signal sequence is not quadrature demodulated into the reproduced symbol sequence which is a correct reproduction of the particular symbol sequence. The data symbol sequence is not adequately reproduced as the reproduced data sequence from the data signal sequence.
A prior patent application Ser. No. 541,690, was filed June 21, 1990, in the United States by Hiroyasu MUTO, the present applicant, based on two patent applications originally filed in Japan under patent application Ser. Nos. 156,856 and 156,857 of 1989. In the manner described in the prior patent application, the inphase and the quadrature phase components are cooperatively representative of a signal vector on a phase plane which is defined by inphase or real and quadrature phase or imaginary axes orthogonally intersecting each other at an origin. The signal vector has a first phase variable on the phase plane around the origin in accordance with the particular symbol sequence and a second phase which is variable according to the data symbol sequence. A sampling clock signal is generated in the receiver to define a sampling interval which is substantially equal to the symbol interval.
In the manner known in the art and described in the prior patent application, the sampling clock signal is used in primarily sampling the inphase and the quadrature phase components of the reproduced symbol sequence into phase samples of the first signal vector. Two consecutive ones of the phase samples have a phase difference which is predetermined in compliance with the particular symbol sequence. In the prior patent application, such phase differences are calculated substantially during the predetermined number of symbol intervals for use in detecting the particular signal sequence in the received signal. Incidentally, the fact is discussed in the prior patent application that the phase samples are subjected to rotation around the origin when the sampling interval is not exactly equal to the symbol interval.
In the manner which will later be described in detail, each of the first and the second phases is subjected to a similar rotation when the local carrier frequency is different from the received carrier frequency. In view of the foregoing, it is possible to briefly say in connection with the quadrature demodulation that the angular modulated signal is demodulated by the local carrier signal into a first signal vector having the first phase and into a second signal vector having the second phase. Incidentally, the particular symbol sequence is said herein to include the partial sequence when a plurality of particular symbol sequences are used in the modulating signal in the manner described in the prior patent application.